Back up your iPhone data first
Check whether Apple is still signing iOS 18 for your device
Download the correct iOS 18 IPSW file for your iPhone model
Install the latest version of Finder on Mac or iTunes on Windows
Connect your iPhone to your computer with a USB cable
Put your iPhone into Recovery Mode
On Mac, open Finder and select your iPhone
On Windows, open iTunes and select your iPhone
Hold Option on Mac or Shift on Windows and click Restore iPhone
Select the iOS 18 IPSW file
Confirm the restore and wait for the process to finish
Set up your iPhone after the downgrade
Restore your backup if it is compatible with iOS 18
If iOS 18 is no longer signed, downgrade is not possible through normal methods